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Gas Supersaturation Thresholds for Spontaneous Cavitation in Water with Gas Equilibration Pressures up to 570 Atm1 Wayne A

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Gas Supersaturation Thresholds for Spontaneous Cavitation in Water with Gas Equilibration Pressures up to 570 Atm1 Wayne A Gas Supersaturation Thresholds for Spontaneous Cavitation in Water with Gas Equilibration Pressures up to 570 atm1 Wayne A. Gerth and Edvard A. Hemmingsen * Physiological Research Laboratory Scripps Institution of Oceanography University of California, San Diego La Jolla, California 92093 (Z. Naturforsch. 31a, 1711-1716 [1976] ; received October 5, 1976) Supersaturations for the onset of cavitation in water were determined for various gases. At am- bient pressure, the threshold supersaturations (in atm) required for profuse cavitation to initiate both in bulk water and at the glass-water interface were CH4, 120; Ar, 160; N2, 190; He, 360. Exposure of the water and its containing surfaces to hydrostatic pressures up to 1100 atm prior to equilibration with gas had no detectable effect on these threshold values. This indicates that pre- formed gas nuclei are not significantly involved. Cavitation at the glass-water interface was investi- gated for CH4, Ar and N, saturations up to 570 atm. For 320 atm saturations, the threshold super- saturation was about one-half that for decompression directly to ambient pressure, and for 550 atm it was about one-third. These results indicate that the dissolved gas concentration is a critical factor limiting the spontaneous nucleation of bubbles in gas supersaturated liquids. Introduction thereby forcing the resolution of such dissolvable nuclei 4. Water that contains large gas supersaturations The results reported here bear directly upon the can remain stable with the absence of cavitation 2' 3. understanding of the gas supersaturated metastable However, as the gas supersaturation in the water is condition in pure liquids, the factors which limit it, increased, the system eventually fails to tolerate the and the characteristics of its degeneration via the supersaturation and bursts profusely with the for- nucleation and growth of bubbles. These results also mation of bubbles. Hemmingsen 3 reported the lar- have implications for the understanding of the for- gest observed gas supersaturation stabilities of water mation of bubbles in organisms with gas super- and found that the minimum supersaturation re- saturated tissues, such as occurs in conjunction with quired for cavitation inception, in the apparent ab- decompression sickness in aviators and divers5'6 sence of pre-formed nuclei, varied substantially with and in tumors supersaturated with oxygen from in- different gases in solution. This threshold super- jection of hydrogen peroxide as an adjuvant to saturation generally increased with decreasing gas radiation therapy 7. solubility, suggesting that the total concentration of gas in solution is important in determining the gas supersaturation stability of liquids. Methods Our efforts to further characterize cavitation nucleation phenomena required examination of this The apparatus (Fig. 1) was designed to first hy- concentration effect in some detail. An experimental drostatically pressurize water, then to equilibrate a system was developed in which wide ranges of gas small portion under high gas pressure, and finally, to transfer it under pressure into an observation pressure could be used to vary the dissolved gas capillary where it could be observed with a binocu- concentrations, necessitating the minimization of lar microscope (15 x) during decompression. All interference by foreign gas nuclei whose persistence chambers and tubing were made of stainless steel. in the water and on container surfaces could vary as Chamber A contained a 2.5 cm thick quartz viewing a function of pressure. This was accomplished by port and a plexiglass lighting port. The pyrex obser- applying large hydrostatic pressures to the water and vation capillary (0.011cm I.D.) was placed inside the container surfaces prior to solution of the gas, this chamber to enable high pressurization of the water and its container surfaces. The capillary had a volume of 0.07 ml and was connected with a * Reprint requests to Dr. Edvard A. Hemmingsen, A-004 plastic sleeve to a steel tube (0.025 cm I.D.) pro- Physiological Research Laboratory, Scripps Institution of Oceanography, La Jolla, CA 92093, USA. truding into the chamber. The pyrex sample bowl 1712 \V. A. Gerth et al. • Gas Supersaturation Thresholds for Spontaneous Cavitation 1712 Screw sample bowl was equilibrated with gas at the desired pressure for at least one hour while being stirred at To Gas Cylinder Capillary 60 rpm with a small Teflon-coated stirring bar. After equilibration, the water was slowly forced into the capillary from the sample bowl by slightly c. loosening the inner screw which maintained the seal kxbvM over the capillary venting device until the sample bowl was nearly emptied. The gas remaining in To Hydrauli* chamber A was replaced isobarically with water -Hp-' PPumu p from chamber B to prevent fogging during decom- Ven, pression. Chamber A was decompressed immediately thereafter while visually observing the capillary for the appearance of bubbles. In one series of experi- ments, the chamber was rapidly (2 — 3 seconds) de- compressed directly to ambient pressure. In sub- sequent experiments, gradual decompressions were used and the pressures at which bubbles were first visible in the capillary were noted. The supersatura- tion tension was then defined as the difference be- tween the equilibration pressure and the latter de- compression pressure. Cavitation was characterized as follows: no cavita- tion — no bubbles formed in the capillary within approximately 2 minutes of decompression; light cavitation — relatively few (1 — 20) bubbles formed, Fig. 1. Schematic diagram of apparatus with enlarged il- leaving most of the water in the capillary bubble lustration of Chamber A showing arrangement of observation free; massive cavitation — bubbles formed effer- capillary, venting device, and sample bowl. vescently. Moderate cavitation characterizes those intermediate cases in which many bubbles formed throughout the capillary, but in which the cavitation had a volume of 1.5 ml and was secured in the was judged to fall short of effervescence. chamber with a plexiglass retainer. The capillary venting device on top of the chamber had a Viton rubber seal retained by a piston and screw assembly. Results Chamber B, with a movable "0" ring sealed plexi- The cavitation behavior of hydrostatically pre- glass piston, could be filled below the piston with pressurized water, gas supersaturated by decom- water, gas equilibrated with only atmospheric pres- pression directly to ambient pressure, is shown in sure. Figure 2. Also shown are experiments in which no Each of the gases used: methane, argon, nitrogen special hydrostatic pre-treatment of the water was and helium; had a purity greater than 99.9%. Com- used. Absence of cavitation with supersaturation mercial bottled distilled water was used without further treatment. Experiments were conducted at tensions of 100 atm or more was possible with each room temperature which usually ranged from 19 C of the gases. The greatest supersaturation tension to 21 C. The capillary was cleaned in concentrated with absence of cavitation was obtained with He at sodium dichromate — sulfuric acid solution and 320 atm. With increasing gas supersaturations, bub- rinsed with distilled water before installation in the bles began to form in increasing numbers. The onset chamber and periodically during the investigation. of cavitation occurred at different supersaturations Capillaries found to have obvious flaws adversely for each of the gases. Light to moderate cavitation affecting the gas supersaturation stability of the always appeared to initiate at or near the glass- water were replaced. water interface. Massive cavitation, however, was Chamber A was filled completely with water with caused by widespread fracturing of the water both the hydraulic pump at the start of each experiment. at the glass-water interface and uniformly in the Hydrostatic pressure from 935 atm to 1100 atm was then applied for a period of at least 15 minutes, bulk water, and was never observed below a certain after which water was slowly forced out the bottom supersaturation tension which was specific for each inlet by gas pressure. The water retained in the gas. The supersaturation threshold for cavitation in \V. A. Gerth et al. • Gas Supersaturation Thresholds for Spontaneous Cavitation 1713 bulk water, i. e. the bulk threshold, required in ex- i. e., the interface threshold, is shown as a function cess of ambient pressure was 120 atm for CH4, of gas equilibration pressure in Figure 3 A. When 160 atm for Ar, 190 atm for N2, and approxi- decompressed from high gas equilibration pressures, mately 360 atm for He. The bulk threshold for He the water tolerated lesser supersaturations before was less well defined than for the other gases due cavitating than when decompressed from lower gas to the rapid expansion of bubbles at the glass-water equilibration pressures. interface. Comparison with data obtained without In Fig. 3 B, the interface thresholds are nor- hydrostatic pre-pressurization of the water (also see malized, for each of the gases respectively, about Ref. 3) indicates that pre-pressurization had no no- their bulk threshold at ambient pressure. Each nor- ticeable effect on any cavitation character except for malized supersaturation (F) is defined as: the supersaturation required for inception of very light cavitation, which in only a few cases was slightly decreased. where Px = the bulk threshold at ambient pressure; Bubbles formed well before ambient
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